The present invention relates to an atomic absorption spectrophotometer using a graphite atomizer furnace method, which analyzes a metal element by heating a sample to be atomized and performing an atomic absorption spectrophotometry, and especially to an atomic absorption spectrophotometer using the graphite atomizer furnace method, by which the analytical accuracy is greatly improved.
FIG. 1 shows a schematic composition of a general atomic absorption spectrophotometer using the graphite atomizer furnace method.
As disclosed in Japan Patent Application Laid-Open Hei 2-259450 or Japan Patent Application Laid-Open Hei 6-58871, a sample 10 to be measured is placed in a graphite tube 2 provided in a graphite atomizer furnace 1, and is atomized by passing current through the graphite tube 2. A light source 3 with a diameter of 3 mm is generally used, and it emits a measuring light 4, including wavelength components of a wavelength range wider than 190-900 nm. The emitted measuring light 4 is converged by a convergence mirror 12, and the image of the emitted measuring light 4 is formed at the central position of the graphite tube 2. In the graphite tube 2, the atomic absorption of the measuring light 4 is caused by the sample 10, and the measuring light 4 which has received the atomic absorption is again converged by a convergence mirror 13 after passing though the graphite tube 2. The image of the converged measuring light 4 is formed at the position of an input slit in an input slit control unit 5. The formed image of the measuring light 4 is controlled by the input slit, and it is led to a spectrophotometer 6.
FIG. 2 illustrates a method of image formation for the measuring light 4 at the input slit. Numeral 20 indicates the input slit, and the quantity of the transmitted measuring light 4 is adjusted by changing the width of the input slit 20. The image 41 is the formed image of the measuring light 4, and its diameter is about 3 mm. The measuring light 4 emitted from the light source 3 has a strong rectilinear propagation property, and its image formed at the input slit 20, has almost the same diameter (3 mm) as that of the light source 3. Strictly speaking, it is the diameter of the image formed by the component of a reference wavelength predetermined as 250 nm. The diameters of the images of other wavelength components are not precisely 3 mm, and these images somewhat blur at the input slit 20. However, the diameter of these blurring images is at most 5 mm. Thus, the length of the input slit 20 is set to 5 mm in order not to decrease the quantity of the measuring light 4, which can pass through the input slit 20.
In the spectrophotometer 6, the measuring light 4 which has passed through the input slit 20 is diffracted, and the component of the required measuring wavelength is output to an output slit in an output slit control unit 11, and further led to a detector 8. The detector 8 converts the illuminance of the detected light to an electrical signal, and outputs the electrical signal to a central processing unit 7. The central processing unit 7 executes the temperature control of the graphite atomizer furnace 1, the current control for the light source 3, the input and output slit control units 5 and 11, and the selecting of the required measuring wavelength component. An input unit 9 sets the heating temperature of the graphite tube 2 when atomizing the sample 10, the required wavelength of the measuring light 4, and the value of current flowing in the light source 3.
In the above atomic absorption spectrophotometer using the graphite atomizer furnace method, since the graphite tube 2 is heated in the measuring operation, the graphite tube 2 itself emit light. Accordingly, the light emitted from the graphite tube 2 is input to the input slit 20 in addition to the measuring light 4. The light emitted from the graphite tube 2 becomes a background component disturbing the atomic absorption spectroscope measurement, and degrades the analytical accuracy of the atomic absorption spectrophotometry.
Therefore, in the conventional atomic absorption spectrophotometry, the background component has been removed by placing a shading plate before the input slit 20 to restrict the light emitted from the graphite tube 2.
However, in the above conventional composition in which the shading plate is additionally placed and fixed, since the measuring light 4 is also restricted by the shading plate at the same time the light emitted from the graphite tube 2 is restricted, the quantity of the measuring light 4 is decreased, which degrades the S/N ratio in the atomic absorption spectrophotometry.
Particularly, in measurements conducted under a low emission strength in the graphite tube 2--that is, measurements using the measuring light 4 with a short wavelength, or measurements with a low heating temperature for the heated graphite tube 2 when atomizing the sample 10, notwithstanding it is almost unnecessary to restrict the light emitted from the graphite tube 2--the light flux input to a detector is decreased by placing the shading plate, which largely degrades the analytical accuracy of the measurement.